The present invention relates generally to solid state gas sensing devices and specifically to a gas sensor using pyroelectric effects.
In conjunction with studies related to high power sources of microwave and millimeter wave radiation, attention has focused on problems in the vacuum tube technology for generating this radiation. Specifically, the cathodes of such vacuum tube devices are sensitive to contamination by residual gases in the tube as well as out-gassing products from tube components as they heat up during operation. In view of the processing of these vacuum tubes (heating to temperatures in the vicinity of 450.degree. C.) rather stringent environmental parameters are placed on any sensor utilized to measure internal contaminants.
Pyroelectric substances are a class of substances which exhibit an induced surface charge when the temperature of the material changes. If a pyroelectric substrate is polarized at a temperature above its Curie temperature, and electrodes are placed on either side of the substrate (across the poling direction) a charge will be developed across the electrodes when the temperature of the substrate is changed. The amount of voltage and/or current will be proportional to the rate of change of temperature.
An article appearing in Sensors and Actuators, Vol. 1, 1981, entitled "Non-FET Chemical Sensors", by Zemel, Keramati, Spivak and D'Amico examines the possibility of utilizing pyroelectric substrates as a gas sensor, and is herein incorporated by reference. The article discusses the anticipated signal from a pyroelectric substrate which is being heated over a period of time. A similar graph of signal output versus time is shown in FIG. 1. The solid line indicates that as heat is applied, the temperature of the substrate changes and this change in temperature provides an output signal from the electrodes on the substrate.
However, if a material is placed on the substrate and begins to undergo a phase change at time T.sub.1 the graph of signal versus time would be as shown in dotted lines. Although the heat input to the substrate is the same as the solid line graph, the heat required to change the state of the material on the substrate would prevent an increase in substrate temperature for some short period of time (until all the material has undergone phase change). Because the temperature of the substrate is not changing at this time T.sub.1, the signal will not increase normally and, in some instances, may actually decrease. This is analogous to the high school experiment in which a Bunsen burner is used to boil water in a thin plastic cup. The cup's temperature rises to that of boiling water and is maintained constant at that temperature until all of the water is boiled away at which time, of course, the cup is heated beyond the temperature of boiling water to its melting point and/or combustion point. However, while the water is boiling away, the temperature of the cup remains at a relatively steady temperature. In our pyroelectric substance, because the substrate is maintained at a relatively constant temperature while the material on its surface is changing phase, the signal output of the pyroelectric substrate is greatly diminished.
Also disclosed in the article is the experimental response of a pyroelectric sensor to the melting of 8 mg of In-Sn solder located on the substrate suggesting that indeed such a pyroelectric sensor was possible. However, theoretical predictions do not necessarily take into account practical realities. It is desirable to be able to differentiate between two or more materials on the substrate surface and thus extremely small substrate temperature increments are desired in order to see the effect of each of two or more materials as they separately absorb heat from the substrate during their melting and/or vaporization. Further, the noise level of a single sensor clouds the sensitivity of the device such that it is difficult to know whether the output is an indication of a material on the substrate, or a random noise signal which has been acquired. Thus, selectivity and sensitivity are problems associated with the experimental pyroelectric gas sensor.
Also known is the use of a pyroelectric substrate as a gas dosimeter as disclosed in U.S. Pat. No. 3,861,879 to Taylor, issued Jan. 21, 1975. Here the exothermic oxidation of carbon monoxide in the presence of a suitable catalyst causes a temperature change in the pyroelectric with a resulting charge redistribution which is sensed.